The present invention relates to a system for transmitting signals between components of a video system over the telephone wiring of a residence.
Until the late 1970's, it was very unusual for ordinary consumers to own electronic devices that generated or supplied video signals. Virtually all video programs viewed on television sets were received "over the air". This situation changed over the past decade as VCRs, video cameras, cable converters, and home satellite systems became popular.
Currently, many consumers are able to watch video programs at different locations because they own more than one television set. When viewing programs from one of the sources mentioned above rather than those picked up "over the air", however, it is necessary to convey the signal from the video source to the television set. When source and receiver are located in the same room, connecting the two with a coaxial cable is usually the easiest method. VCRs and cable converters are nearly always connected to nearby television sets in this manner.
When the source and receiver are not located in the same area, a network of coaxial cabling extending through the residence is a fine solution. Most residences, however, are not wired this way, or have networks that do not allow access at all desired locations. Furthermore, most consumers insist that the wiring be neatly installed or kept entirely out of sight, making installation of a network very difficult and unwieldy. This presents a problem when connection between a video source and a television requires wiring that extends between rooms, especially rooms located far from each other, or on different levels.
Today, it is very common for a residence to include a VCR and a television located in a "sitting room", and a second television located in a bedroom. This has generated an enormous demand for technology that transmits video across a residence without requiring installation of new wires. Possible solutions are to broadcast the signal at low power, or to use power lines or telephone wiring, which are always available, as a conductive path.
Broadcasting is currently not feasible in the U.S. because of FCC regulations, and is not feasible in most other countries for similar reasons. (Several consumer devices that broadcast video at low power have been marketed, however, despite their clear violation of FCC regulations. This testifies to the existence of a large demand for transmission of video over short distances.) In addition to legal obstacles, the possibility of unintended reception of broadcast signals outside residences, and the possibility of interference from other sources broadcasting at the same frequency also present problems.
Regulations covering transmission between source and receiver over conductive paths are much less restrictive, and signals transmitted by this method are much less likely to encounter interference from other signals or be open to interception. Transmission across power wiring is very difficult, however, because appliances typically attached to those networks often impart electrical noise at many different radio frequencies, creating a high potential for interference. Furthermore, a reliable conductive path is not always available across "fuse boxes", causing problems when source and receiver derive power from different circuits.
The difficulties in transmitting video by broadcasting or by conduction over power lines leave conduction over telephone wiring as the sole remaining option. This technique also involves very serious technological and legal challenges, however, and no solution has been found.
The most obvious difficulties are avoiding interference with telephone communications and conforming with all regulations that govern devices that connect to the public telephone network. Because telephone wiring in the US and many other countries typically includes four conductors, only two of which are used for communications in residences served by a single telephone number, availability of the unused pair would seem to present an interesting opportunity for avoiding these problems. Unfortunately, wiring installers often do not connect the unused pair at the network junctions, leaving breaks in the conductive paths offered by these wires.
The path supplied by the active pair, on the other hand, is guaranteed to be continuous between two jacks as long as telephone devices become active when connected at those jacks. An exception is residences where each jack is wired directly to a central electronic switching unit that provides an interface to the public telephone system. The conductive paths between jacks are likely to be broken across this unit.
There are other technical and legal problems associated with transmission over this wiring beyond those created by the connection to a public or private telephone network. The technical problems derive from the fact that transmission of video was not a consideration when standards for wire properties, installation and connection techniques, and telephone electronics were established. Because these are all factors that can influence the ability of the wiring to reliably transmit high quality RF signals, this environment is poorly suited for transmission of video.
Further legal problems derive from the fact that all RF signals conducted across unshielded wiring will broadcast at least some electromagnetic radiation. (Unlike coaxial cable, telephone wiring is not shielded by a grounded metallic conductor that eliminates radiation.) Because restrictions on RF radiation are very limiting in the US and most other countries, they can potentially defeat any particular electronic technique that could otherwise successfully achieve transmission.
Systems have been developed to transmit video signals over ordinary telephone wiring, but none is practical for the residential application described. Chou (U.S. Pat. No. 4,054,910) discloses a system for transmitting video over an ordinary pair of wires without boosting the video signal in frequency. Video signals transmitted by devices that follow that design, however, would include energy at low frequencies that would interfere with telephone signals.
Tatsuzawa (U.S. Pat. No. 3,974,337) discloses a system that slightly boosts video signals in frequency (by approximately 0.5 Mhz) to prevent conflict with voiceband communications. The system also requires, however, a sophisticated procedure for compressing the bandwidth of the signal so as to avoid use of energies at the higher frequencies, which attenuate quickly. Further, the higher end of the resulting band is "preemphasized", or amplified more than the lower frequencies, in order to account for the remaining differences in attenuation.
The purpose of the technique disclosed by Tatsuzawa is to allow video signals to travel distances on the order of 1 km or more. The electronics that reduce and expand the signal bandwidth however, are very expensive. There is also a major difficulty in that the preemphasis of the signal must be adjusted depending on the distance between source and receiver. This is of significant inconvenience to a consumer. Further, the system depends on electrical characteristics particular to frequencies between 0 and 4 Mhz, limiting the transmission frequency to that band. This creates legal problems because in the U.S., for example, regulations severely limit the RF energy below 6 Mhz that can be fed to wiring that is connected to the public telephone network. Finally, the restriction to a single band allows for transmission of only a single signal.
There are countless methods for reducing the resolution or the refresh rate of a video signal in order to reduce the bandwidth enough to avoid the problem of attenuation, e.g. Lemelson (U.S. Pat. No. 4,485,400). Current video standards in the U.S. and elsewhere, however, use a refresh rate just quick enough to avoid annoying "flickering" of the picture. Because most consumers have little tolerance for "flickering" or a reduced picture quality, these techniques do not present solutions to the problem at hand.
Two commercially available devices are known by the inventors to transmit uncompromised video across telephone wiring. The first device is marketed by several cable equipment supply companies, e.g. the J411 system marketed by Javelin Electronics of Torrance, Calif. The list price of this device is nearly $1000.
The device transmits a single unmodulated video signal across the wiring. Because some of the energy of these signals is concentrated at frequencies below 3 khz, the device will cause interference with telephone communications. Further, the specifications stipulate that "transmission must be via dedicated twisted pair (of which telephone wiring is a subset) . . . and must be clean, unloaded, and unconnected to any other device." The device also "pre-emphasizes" the signal by imparting more amplification at the higher frequencies, adding expense and the inconvenience of requiring adjustment on the part of the user.
The second device, "Tele-Majic," is marketed by Impact 2000, a catalog specializing in consumer electronic devices. This device is composed of a pair of identical connecting cables. These cables are advertised as enabling one to connect a video source to a residential telephone line in one area, and a television receiver in a second area, for the purpose of viewing the source at the second location.
Each cable consists of a classic matching transformer which connects to the video devices, a capacitor for blocking telephone signals to prevent interference, and a telephone cord terminated with a "male" RJ-11 plug, the standard plug for connection to a telephone jack.
The device is intended to work by simply feeding the video signal from the source on to the wiring, and recovering it at a remote location. For several reasons, it does not nearly solve the problem at hand.
To begin with, because "Tele-Majic" does not provide a video amplifier, the strength of the signal fed to the wiring will be limited by the strength of the signal supplied by the source. This causes a problem because the output signal levels generated by VCRs sold in the U.S. are limited by law to approximately 10 dB re 1 mV into 75 ohms. At this level, the video signal can transmit only a few feet before the wiring will attenuate its energy below the level required for quality television reception.
Beyond the limitations caused by low signal power, the matching transformer of the "Tele-Majic", which constitutes half of the electronics in the device, is significantly suboptimal, and does not teach anything about the correct purpose of that component. In an apparent attempt to economize, the common 75 ohm/300 ohm matching transformer, built to connect between 75 ohm coaxial cabling and "twin-lead" wiring was chosen. Because matching transformers of the same design are included with virtually every video device sold in the U.S., these are extremely inexpensive to obtain.
A matching transformer can serve the purpose of matching the impedance of video equipment to telephone wiring. The impedance of typical telephone wiring, however is approximately 100 ohms at low VHF channels, not 300 ohms. This will create an impedance mismatch, and video signals will lose more energy than is necessary when passing from the source onto the network via this cable.
The transformer can also serve the purpose of balancing the voltages on the two leads of the telephone wiring, in order to reduce electromagnetic radiation. Because the transformer used by "Tele-Majic" is designed to handle signals at all video frequencies, however, it cannot balance the video signal nearly as well as a transformer specifically tailored for a specific frequency. The lack of balance will cause more radiation than would be released by a maximally balanced signal.
Another problem is that complete isolation of telephone signals using the particular transformer supplied with the device requires two capacitors rather than the single one which comes with "Tele-Majic". This design flaw will cause total disruption of telephone communications when the device is connected to a coaxial port whose outer shield connects to ground.
Given the ability to transmit video signals throughout a residence, the viewer of signals at a remote television remains limited in the ability to control the apparatus that supplies the signal. Many video sources, especially VCRs and cable converters, are designed to cooperate with hand-held controllers that send out infrared control signals upon command of the user. Unfortunately, signals from these devices do not travel between rooms unless there is a line-of-sight path between transmitter and source. It follows that a significant demand for transmission of control signals should arise as a result of technology that succeeds in transmitting video across telephone wiring. Furthermore, there is an obvious economy in achieving this transmission using the same wiring used for transmitting video.
Robbins (U.S. Pat. No. 4,509,211) discloses the only known method for transmitting control signals from an infrared transmitter over a transmission line that also is used to transmit video signals. That method converts the infrared signals received in the area of a television to electrical impulses, which, due to the nature of typical infrared control signals, are concentrated at frequencies below 1 Mhz, lower than typical video frequencies. Those impulses are transmitted across the transmission line to the area of a programmable video source, where they are converted back to infrared energy, recreating the original light pattern.
The technology taught by Robbins, however, is not adequate for situations where the energy of other signals sharing the transmission line is concentrated at frequencies that fall within the frequency bands that confine the control signal energy. This is the case when active telephone wiring serves as the transmission line. Under the method Robbins discloses, signals from infrared controllers will conflict with telephone communication signals because they both have information content at frequencies between 0 and 3 khz. Any receiver that is tuned to frequencies between 0 and approximately 3 khz, such as a telephone set, will react to both telephone signals and control signals. Either telephone communications will be noisy, or the infrared signals will be ambiguous, or both. (If one signal is much stronger than the other, that signal may be received without distortion.) Furthermore, the system will fail whether or not video signals are present.
Robbins discloses devices that include, in combination with other technology, "isolation circuitry" which prevents the electrical signals derived from infrared light patterns from reaching the video source and the television receiver. Robbins teaches that "power lines, telephone lines or other existing conductor systems can be used, providing the various signals do not interfere, or providing isolation means are provided." This is incorrect. If two signals overlap in frequency, no isolation means will cleanly separate them so that only the desired signal reaches the receiver that is designed to react to it.
Indeed, the isolation circuitry disclosed is totally unnecessary even for the very application that is the focus of the Robbins patent. Under the system Robbins discloses, video signals and control signals transmit across a single conductive path at non-overlapping frequencies, and isolation circuitry is provided to block the control signals from the video source and the television receiver connected to this path. Because VCRs and virtually all other video sources have reverse isolation provided at their output ports, electrical energy incident at these ports will have no effect at all, and extra isolation is not required. Further, when a television is tuned to a particular video channel, signals at frequencies outside of that channel are ignored unless their energy level is very high. The control signals will be ignored in this manner, just as video signals at VHF channel 3 and VHF channel 5 are ignored by a television receiver tuned to VHF channel 4.
Beyond Robbins' incorrect teaching of isolation circuitry and the fact that the infrared transmission system he teaches is inadequate for the present application, Robbins teaches nothing regarding transmission of video over telephone wiring.
An electronic transmitter/receiver pair called the Rabbit follows the electronic principles disclosed in Robbins' patent to send video and infrared signals between a VCR and television. This device, which cites the Robbins patent on its packaging, has been available at retail outlets since 1985. It uses a transmission line composed of a single very thin insulated wire pair which must be installed by the user between the VCR and a television. Thus, it embodies the very difficulty that the current invention seeks to address.
There is another system known for transmitting infrared signals from a television to a remotely located VCR, but it differs in that it uses broadcast technology rather than a transmission line. Called the "Remote Extender" and marketed by Windsurfer Manufacturing of DeFuniak Springs, Fla., this device converts the infrared signals to electrical impulses, then boosts these impulses to a UHF frequency and feeds them to an antenna from which they broadcast. A remotely located receiver picks up these UHF signals, downshifts them back to their original frequency band, and uses the resulting impulses to recreate the original infrared pattern.
Because this system uses broadcast technology, it is much more susceptible to interference, and its receiver has the potential of mistakenly picking up control signals from the transmitter of a second transmit/receive pair operating nearby. Furthermore, it is obviously more economical to use the telephone wiring for transmitting control signals when combining with technology that transmits video using that medium.
The simultaneous transmission of infrared control signals and a single video signal across telephone wiring is the major focus of the technology disclosed herein. It is easy to see, however, the usefulness of extending this technology to allow signals from more than one video source to transmit at a given time.
When each source transmits a signal at a different frequency band, the telephone wire medium should present no barrier to the use by multiple sources. Many factors, however, limit the number of bands that are available. An especially restrictive limit, of course, is imposed by the difficulties of using telephone wiring as a medium. In the event that the number of desired sources exceeds the number of available channels, this limit becomes restrictive.
If a viewer can disable all but one of multiple sources that use the same band, however, the picture from the remaining source can be displayed without interference. This possibility creates a demand for a technique that allows a user to quickly, conveniently, and remotely activate one of several sources that are connected and ready to transmit.